The present invention relates to the field of marine seismic data acquisition. More particularly, the present invention relates to a tuned bubble attenuator, for use with a seismic source towed behind a vessel, for modifying the side directional frequencies produced by the seismic source.
Seismic exploration in water is performed by selectively introducing acoustic pulse energy into the water. The water can be located in marshes, tidal areas, lakes, rivers, bays, or open seas. Acoustic pulse energy is typically propagated by air guns towed by a vessel and is reflected from different reflective horizons. The reflective horizons can comprise the interface between the water and soil or the interfaces between subfloor geologic formations. The reflections are sensed by towed receivers such as hydrophones, and the resulting hydrophone signals are processed to permit evaluation of the geologic formations.
The depth of investigation and signal quality generated by the acoustic source depends on various factors. The ratio of signal strength to noise level at the hydrophones is a significant factor in seismic data acquisition. To improve this ratio, U.S. Pat. No. 4,625,302 to Clark (1986) described an apparatus for generating an "acoustic lens" formed by air bubbles in a water zone. The acoustic lens refracted acoustic waves, reflected from the sea floor or from subfloor geologic structures, to reduce undesirable noise sensed by the hydrophones.
U.S. Pat. No. 4,618,024 to Domenico (1986) similarly described a gas releasing device for forming a paraboloid in the water for the purpose of reflecting acoustic wave energy downwardly into the water. The paraboloid attenuated upwardly and horizontally traveling acoustic wave energy produced by the seismic source as the source was moved through the water. Domenico emphasized that the paraboloid structure is important to the objective of downwardly reflecting acoustic source energy, and that the paraboloid bubble structure effectively reduced the size of the acoustic energy source.
The strength and quality of the acoustic source pulses is important to the depth of investigation and reflected signal quality. Because the acoustic pulse energy is transmitted in all directions within the water, the strength of the acoustic pulse energy propagated downwardly into the geologic formations represents only a portion of the acoustic source energy. Much of the acoustic pulse energy is dissipated laterally and upwardly in the water. Although bubble zones are known to modify acoustic pulse propagation in a static, nonmoving environment, the beneficial effect of bubble zones towed by a moving seismic vessel does not effectively permit noise attenuation over a wide frequency range.
Accordingly, a need exists for an improved apparatus and method for reducing undesirable noise and for enhancing the effectiveness of acoustic pulses as a seismic vessel tows seismic energy sources through the water.